The process of the present invention involves the preparation of aqueous polymer solutions having special utility as drive fluids and/or mobility buffers in the secondary or tertiary recovery of oil from subterranean oil-bearing formations or reservoirs. Such solutions are generally prepared from a water soluble, ethylenically unsaturated monomer containing at least one vinyl grouping wherein the vinyl is an acrylyl, a vinyl cyanide, a styryl, and water soluble salts thereof. When the vinyl is an acrylyl it may be represented by the formula: EQU CH.sub.2 .dbd.CY--CO--X
wherein X is hydrogen, an amino group (NH.sub.2), a hydroxy group, a methyl group or an OR group wherein R is a lower alkyl group, and wherein Y is hydrogen or a methyl group. Examples of suitable monomers include acrylamide, acrylic acid, acrylonitrile, methacrylic acid, methacrylamide, methacrylonitrile, methyl methacrylate, and sodium styrenesulfonate. Of this group of monomers, acrylamide is preferred. Polymerization of the monomer desirably is carried out in an aqueous solution containing about 1 to about 20 weight percent, preferably about 2 to about 12 weight percent, and most advantageously about 6 to about 8 weight percent monomer. The pH of the solution generally will be about 4 to about 11, usually about 6 to about 9. The temperature of the polymerization reaction can range from about 5.degree. C. to about 100.degree. C., with a temperature of about 20.degree. C. to about 65.degree. C. being preferred. The time required to complete polymerization of the monomer can vary from about 2 to about 20 hours. With lower concentrations of catalyst, polymerization times can be effectively reduced.
Since oxygen inhibits the polymerization reaction, it is desirable to limit the amount of free oxygen present to less than about 0.1 parts per million. Polymerization of the monomer is carried out in the presence of a free radical generating initiator. Exemplary of initiators suitable for this purpose are azo compounds such as azobisisobutyranitrile and azobisisobutyramidine chloride; peroxides such as hydrogen peroxide, sodium peroxide and benzoyl peroxide; alkyl and dialkyl peroxides such as, for example, t-butyl hydrogen peroxides and diethyl peroxide; alkali metal and ammonium persulfates including sodium persulfate, potassium persulfate and ammonium persulfate; and alkali metal bisulfites exemplified by sodium bisulfite and potassium bisulfite. Compatible mixtures of the foregoing may be used. In those instances where the monomer to be polymerized is an acrylamide, ammonium persulfate is the preferred free radical generating initiator.
The concentration of the free radical generating initiator used in the polymerization of the monomer usually will be about 50 ppm to about 500 ppm, preferably about 150 ppm to about 300 ppm, based upon the weight of the monomer in the polymerization reaction mixture.
A preferred polymer solution for use in the secondary and tertiary recovery of oil from an oil-bearing formation is an aqueous solution of a partially hydrolyzed polyacrylamide. Accordingly, following polymerization of the polymer, a hydrolyzing agent is added to the aqueous polymer solution. An especially effective agent for this purpose is a 50% solution of sodium hydroxide. The amount of hydrolyzing agent introduced into the polymer solution desirably is sufficient to hydrolyze approximately 20% to 40% of the amide groups comprising the polyacrylamide. One of the by-products of the hydrolysis reaction is ammonia which is generated in a molar amount in substantially direct proportion to the amount of hydrolyzing agent employed. More specifically in this connection, if 30 moles of base are used to hydrolyze the polymer, approximately 30 moles of ammonia will be produced during the hydrolysis reaction. As a result the pH of the aqueous solution of the partially hydrolyzed polyacrylamide is increased, usually to a level of about 10 to 12, or higher.
Generally speaking, the concentration of the partially hydrolyzed polymer in the aqueous solution will be about 6% to about 7%, and the solution will have a viscosity of about 450,000 cp to about 550,000 cp. It is necessary, prior to final dilution and injection, for example, into an input well of a subterranean oil-bearing formation or reservoir, to dilute the aqueous solution of the hydrolyzed polymer to provide a concentration of the polymer in the solution of about 1% to about 2%. Dilution of the hydrolyzed polymer solution advantageously is achieved by passing the solution through a series of dispersing stations which act to progressively increase the surface area of the polymer. By thusly continuously exposing previously unexposed surface areas of the polymer to water, the take-up or absorption of water by the polymer is appreciably enhanced, and dilution can be effectively carried out in a matter of minutes. Apparatus especially suitable for this purpose is disclosed in U.S. Pat. No. 4,402,916. However, it has been found that when the apparatus is used with water of the type available at most oil fields, scaling occurs on the perforated members comprising the dispersing stations of the apparatus. Scaling, in the main, is due to the deposition of calcium carbonate present in the water employed as the diluent for the aqueous polymer solution. The build-up of such deposits on the perforated members reduces their ability to increase the surface area of the polymer thereby lessening the efficiency of the apparatus and eventually plugging the unit.